Book/Dissertation / PhD Thesis

FZJ-2019-05145

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Entwicklung von Reparaturmethoden für Nickel-Superlegierungen mittels thermischer Spritzverfahren

Kalfhaus, T. (Corresponding author)FZJ*

2019
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-95806-418-8

Please use a persistent id in citations: http://hdl.handle.net/2128/23298

Abstract: The outstanding high temperature properties of Ni-based superalloys are the reason for their application in modern gas turbines. The development towards a singlecrystalline microstructure lead to a significant increase in efficiency over the past decades. Repair procedures with the potential to reduce the overall cost of these expensive components are of special interest for the gas-turbine industry. The highquality standards and the poor weldability of Ni-based superalloys limit conventional repair procedures. Thermal spray techniques are a promising method for repair applications due to their outstanding coating properties. This thesis is engaged with the potential for a possible repair application of Ni-base superalloys using vacuum plasma spray (VPS) and cold gas spray (CGS). A focus of this thesis is on the single crystalline repair of the alloy CMSX-4. A precise measurement and control of the substrate temperature during both spray techniques is important to influence the microstructural properties of the coating. Therefore, a heated substrate holder with a maximum substrate temperature of 1000 °C was constructed and build. The first part of the thesis considers the influence of an increased substrate temperature on the VPS-coatings. An epitaxial crystal growth of the polycrystalline repair coating is observed in sections on the polished single-crystalline substrate. A parameter study was used to decrease the oxide content within the coatings. The specific movement of the polycrystalline repair coatings into a hot-zone leads to directional annealing, resulting in a columnar microstructure. With the aim of stimulating grain growth from the SX substrate, a FEM-thermal-field evolution of a Bridgman furnace was simulated. The results indicate conditions within the sample to be adequate to allow hot-zone movement through a thick polycrystalline VPS-repair coating originating in the SX-substrate. In the second part of the thesis it is shown that the maximum gas temperature of the CGS-system is not sufficient to deposit CMSX-4. Therefore, the focus for CGS-repair was changed to the polycrystalline repair of IN-738 using the four alloys IN-625, IN-713, IN-738 and Rene’80. The residual stress of the coatings is characterised using a hole-drill and a curvature approach. To improve the coating thickness the elastic stored energy of the coatings is reduced using the heated sample holder and laser structuring results in a better adhesion between the substrate and the coating.It was found that subsequent HIP-heat-treatments decrease the porosity with an increasing annealing temperature. The strongly deformed microstructure of the CGScoatings attached to aged substrate can be rejuvenated with HIP-heat-treatments atoptimized annealing temperatures. Simulated Stress-Strain-Curves using Indentation data revealed similar strength properties of the IN-738-repair coating in comparison to the IN-738-substrate.

Note: Dissertation, Univ. Bochum, 2019

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Contributing Institute(s):

1. Werkstoffsynthese und Herstellungsverfahren (IEK-1)

Research Program(s):

1. 899 - ohne Topic (POF3-899) (POF3-899)

Appears in the scientific report 2019

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